A composite is a resin or resin-like crystalline, amorphous or semi-crystalline matrix in which is embedded wires, fibers, whiskers or flakes typically of carbon, graphite, fiberglass or boron. Reinforcing materials can be long, short, layered, chopped, orderly or random. Typically, layers composed of parallel fibers are oriented and laminated in different directions to produce a stress-free, lightweight, uniform sheet of unusual strength.
Reinforced polymer composites can maintain material strength and integrity at continuous source temperatures typically at 400.degree. F. and higher and have substantial usage in the fields of aircraft, automotive structures, construction materials, machine parts and a variety of consumer product constructions as a replacement for metal and wood.
A major problem which has plagued the plastic and plastic matrix composite industry is the lack of appropriate technology for joining of these composites. Present state-of-the-art bonding methods are unsatisfactory; these include adhesives, resistance welding, ultrasonic bonding, vibration welding, induction bonding, high frequency welding, mechanical fasteners and infrared radiant heat. Each method has inherent problems which this invention overcomes.
Adhesives are difficult to apply uniformly and, because of the difference in composition and lack of molecular continuity, create fracture lines.
Resistance welding requires conductive metal or carbon strips that remain in the joint after the resistance current is applied. This weakens the bond by producing a discontinuity in the integrity of the plastic.
Ultrasonic bonding is effective only in layers less than 0.040" and is suitable for short bonding lines of under 10" in length. The high frequency vibrating of the material inherent in the ultrasonic bonding process dislocates and abrades the fiber structure of the composite material resulting in a weakening of the material.
Vibrational bonding requires the rapid frictional abrading of the two surfaces to be bonded which disrupts the fibers imbedded in the matrix.
Induction bonding requires the use of ferrous metal powders which are left in the joint area of the interface of the two parts being bonded thus disrupting the integrity of the composite plastic material and requiring under bond lines to accomplish the joining.
High frequency welding results in a weak bond and thinning of the material at the joint because the materials typically used in the thermoplastic matrix are dielectrically resistant to high frequency current. If the reinforcing fibers are dielectrically conductive, then transfer of heat to the interior of the composite material results in overheating, melting, thinning and weakening.
Mechanical fasteners are unsatisfactory because the integrity of the composite sheet is interrupted by the puncture necessary to affix the fasteners. Mechanical joints often leak and create a weakness of the joints.
Infrared and laser heating prior to the present invention has been unsuccessful because the electromagnetic energy is absorbed and dissipated by the material surface. The bonding surface, when exposed to unfocussed radiant energy, melts, deforms and weakens. When removed from the heat the material cools before it can be brought together. One of the novel features of the present invention is the intermittent application of a narrow line, point or series of points of focussed infrared heat at the bonding interface of each sheet or part. It is the periodicity of the intense heat which resolves the problems previously inherent in infrared and laser heating. In accordance with the method and apparatus of the present invention, means are provided to effect such periodicity and to hold the parts apart while heat is applied to the bonding surfaces and to quickly join the parts after heating before cool down.
It is an object of the invention to provide a quick and effective method of bonding plastic matrix composites and melt fusing monolithic thermoplastics and elastomers which overcomes virtually all of the problems inherent in present state-of-the-art bonding processes and results in bonds and fusions which consistently test in excess of 2,000 psi in lap shear, 1,000 psi or more in flat pull strength, and over 5 lbs. in peel.
It is a further object of this invention to provide a uniform bond which avoids the discontinuity introduced by adhesives; avoids the vibration and tearing inherent in ultrasonic bonding; avoids the ferrous residue inherent in induction bonding; avoids the thinning and weakening produced by high frequency welding; solves the problem of material puncture which is a problem with mechanical fastening; and eliminates the installation of high resistance wire that remains in the seam during resistance welding.
It is an object of this invention to provide a strong, stress-free bond of any geometric configuration.
It is an object of this invention to provide a leak-proof homogenous bond line without any reduction of the composite sheet's or part's thickness by means of molecular bonding within the resin of the joined thermoplastic composites such that the bond is as strong as the resin itself.
It is a further object of this invention to provide a method of bringing together the surfaces to be bonded immediately on the cessation of heating which result in bond strengths up to and in excess of 2,000 psi in lap shear on a 1".times.1" bond line, and over 1,000 psi in a flat tensile pull, and more than 5 lbs.in a test peel on the same 1".times.1" bond specimen.
It is a further object of this invention to provide non-contact heating of bond line material without reducing the thickness of the pre-joined sheets or parts in substantially reduced times of approximately 30 seconds to 10 minutes.
It is a further object of this invention to provide intermittent focused radiant heat means to both surfaces to be bonded prior to bonding and to provide heat means to the enriching matrix prior to bonding whereby only the outermost layers of the designated bonding areas on surfaces to be bonded are heated, leaving the internal temperature and fiber structure of the material essentially unaffected, and leaving the adjacent surface material unheated.